Amplifiers are commonly used to provide gain to an input signal. For example, if a voltage amplifier has a voltage gain of 10, then an input signal of 50 millivolts (“mV”) applied to the voltage amplifier results in an output signal of 500 mV. An amplifier typically has a range in which the amplifier operates in a linear manner. For example, a voltage amplifier connected to a 5-volt power supply may be linear for outputs up to 4.5 volts. However, driving output voltages greater than 4.5 volts may force the amplifier into non-linearity, resulting in overload of the amplifier and thus potential inaccuracy. In addition to linearity and noise concerns that can affect accuracy, many amplifier circuits can also be susceptible to input offset. For example, even an input offset of a few millivolts can greatly affect the accuracy of the amplifier circuit.
Many amplifier circuits, such as instrumentation amplifiers, generate a current signal between two amplifier devices through a direct connected resistor. For a direct connection of the resistor between the amplifier devices using DC coupling techniques, such amplifier circuits can generally operate without difficulty so long as the gain of the amplifier circuit is low. However, if the gain in the amplifier circuit is high, then large offset can occur to decrease accuracy of the amplifier circuit.
To address large voltage offset, AC coupling is often implemented, such as through capacitive coupling with the addition of a capacitor configured between the amplifier devices. In many instances, the capacitor is provided external to the integrated circuit chip containing the amplifier circuits, particularly if the capacitor exceeds 200 pF in value. As a result of an additional bond pad that is utilized and the accumulated stray capacitance, mainly due to the need to configure the bond pad to allow for the connection of the external capacitor, the capacitive loading of the external wiring connections becomes unequal. When the capacitive loading becomes unequal, the ability to achieve low, even harmonic distortion and/or operate within the linear region becomes extremely difficult to obtain.
Unfortunately, even in an instance when the amplifier circuit utilizing AC coupling is able to operate within the linear region, the AC coupling can still cause difficulty in the recovery time of the amplifier circuit, such as difficulty in the identification of a closely-coupled in time small signal following a large pulse signal. Moreover, in high-gain, if the voltage offset is too high, the voltage offset may be amplified to such unacceptably large values that AC coupling techniques cannot restore the amplifier to proper function. Thus DC coupling techniques may be more desirable in many instances due to the close signal interaction and improved recovery time. Unfortunately, conventional DC coupling techniques are limited in their ability to eliminate input offset voltage.